1. Field of the Invention
The present invention relates to a head position determination control apparatus used in a data recording/reproducing apparatus such as a hard disk unit.
2. Description of the Related Art
In a conventional hard disk unit, a plurality of recording media are axially arranged at predetermined intervals. Part of one recording medium constitutes a servo surface, on which servo data necessary for servo control of a magnetic head is recorded, and all other recording media constitute data surfaces. The magnetic head consists of a plurality of data heads corresponding to the respective data surfaces, and a servo head corresponding to the servo surface.
In a dedicated servo system, each data head is sought to a target track of a corresponding data surface, in accordance with servo data recorded on the servo surface and read therefrom by the servo head, and is positioned at the center of the target track by position determination control. In contrast to such a dedicated servo system as described above, a composite servo system using both a dedicated servo system and a data surface servo system has been developed. Since this composite servo system makes use of servo data recorded on the servo surface and servo data recorded on part of each data surface, each data head can be positioned on a target track of a corresponding data surface with high precision. More specifically, in the above composite servo system, the incidence of positioning errors caused by a thermal offtrack phenomenon--i.e. a data head positioning error vis-a-vis a corresponding surface, due to changes in the ambient temperature of the hard disk unit--can be reduced. The thermal offtrack phenomenon is known, in which a positioning error of each data head on a corresponding data surface occurs due to changes in ambient temperature of the hard disk unit. Since, in the composite servo system, servo data relating to each data surface can be obtained, therefore position determination control can be performed in respect of each data surface. However, when the system is placed in a head switching mode (a mode in which the read/write operation is switched from a first data head to a second data head), head position determination control must be performed anew when the second data head is to access the same target track as was previously accessed by the first data head. In contrast, in the dedicated servo system, when a read/write operation is to be performed with respect to different data surfaces of the same cylinder, the read/write operation executed in a continuous manner, without the need for fresh head position determination control to be performed after the head switching mode is executed.
As systems for recording servo data on the data recording surface, there are a sector servo system in which the servo data is recorded in the individual sectors of the data track, an EMBEDED servo system in which the servo data is recorded for every several sectors in the respective tracks, an index servo system in which only one servo data is recorded in each of the tracks, and the like. Hereinafter, the system for recording the servo data on the data recording surface is provisionally called a data surface servo system. In such data surface servo system, the servo data cannot be continuously obtained. For this reason, when positional correction is performed by using the data surface servo data, servo data of, e.g., several sectors are required. The positions of the heads on the respective data surfaces do not always coincide with each other due to the thermal offtrack phenomenon. Therefore, when heads are switched, head positioning must be controlled again. In order to set a head in an ontrack state with respect to a new track, servo data of several sectors must be read to perform positioning. As a result, the time required for position determination control (settling time) is increased.